and then I lookup his username in the db and create a user object with all necessary data (team, name, permission level, role, ...) from the return data.
Then I keep this object in a session var until the user logs out.

This was working like a charm locally but now that I've deployed to the server I get

defaultAppPool

for every user.

What am I doing wrong?

In my web.config file I have:

authentication mode="Windows"

I have to admit this is the first time that I'm doing the deploying myself and everything went smoothly except for this.